This invention relates to a motor having motor parts which are movable relative to one another, of which the first motor part forms a multi-pole excitation field in an air gap by means of a permanent magnet with 2p magnet poles and of which the second motor part is a yoke having a number T of pole teeth which project towards the air gap and which carry exciter coils and are spaced by pole gaps.
Such motors are known as rotary motors, for example, from DE 28 23 208 C2 and from DE 31 22 049 C2. In such motors a detent torque is produced as a result of the interaction between the magnetized poles of the permanent magnet and the pole teeth of the yoke. This detent torque is usually undesirable because it may give rise to unwanted oscillations. The order of the fundamental frequency of the torque follows from the least common multiple of the number of magnet poles 2p and the number of teeth T and should be as large as possible because the amplitude of this undesirable torque decreases as the order increases. Accordingly, there are motors having large detent torques and motors having small detent torques on account of the number of magnet poles 2p of the permanent magnet and the number of pole teeth T. A motor having a pole-pair number 2p=2 and a tooth number T=3 will have a detent torque with a fundamental frequency of the order 6. The amplitude of the detent torque is consequently comparatively high. DE 28 23 208 C2 describes how such detent torques can be reduced by indentations in the pole teeth.
However, there are also motors which on account of their magnet pole number 2p and tooth number T already have a small detent torque without such additional measures. For example, the detent torque of a motor having a magnet pole number 2p=8 and a tooth number T=9 has a fundamental frequency of the order 72. Therefore, the amplitude of the detent torque of such a motor is very low. However, a problem associated with such a motor is that it is not symmetrical, i.e. the magnetic field in the air gap is not symmetrical. As a result, radial forces are exerted on the rotating motor part of the motor. For example, if a magnet pole of such a motor directly faces a pole tooth across the air gap, the magnet pole which is 180.degree. shifted in the circumferential direction faces a pole gap, as a result of which the radial forces between the two motor parts no longer cancel one another.